The present invention relates to an optical storage medium for which a light beam, such as a laser beam, is used in data recording or reproduction and a method of producing such an optical storage medium. This invention, particularly, relates to a write-once type optical storage medium having a recording layer provided in the vicinity of the surface of either side of the medium and a method of producing such an optical storage medium.
DVD has been the mainstream of optical storage media for data recording or reproduction with a laser beam. Another type of optical storage medium becoming popular is Blu-ray Disc (referred to as BD hereinafter) having a larger storage capacity than DVD.
DVD and BD have the same disc shape with 12 cm in diameter and 1.2 mm in thickness. The differences between DVD and BD are as follows: DVD has a recording layer located at almost the middle of the thickness, or 0.6 mm away from both disc surfaces. Recording/reproduction is performed to the DVD recording layer with a laser beam at a red wavelength of 650 nm. BD has a recording layer located at 0.1 mm from either of the disc surfaces. Recording/reproduction is performed to the BD recording layer with a laser beam at a blue-violet wavelength of 405 nm.
Higher recording density is achieved for BD with a shorter wavelength for a laser beam and a larger number of numerical aperture (NA) for an objective lens than DVD.
A typical BD has at least a reflective film, a first dielectric film, a recording film, a second dielectric film, and a cover layer formed in order on a resin substrate of 1.1 mm in thickness. The total thickness is 0.1 mm except for the substrate.
The inventors of the present invention have found that BDs now on the market suffer deterioration of reproduced signals when left in a hot and humid environment for certain hours.
The test in which optical storage media are left in a hot and humid environment is one of the measures to evaluate the long-term reliability of the optical storage media in a short time. In this test, optical storage media are usually left in an 80° C. and 85% RH environment for 96 hours, which corresponds to a normal environment of 25° C. and 30% RH for several ten to several hundred years. The long-term reliability can be evaluated with measurements of average SER (Symbol Error Rate); the higher the average SER, the more the reproduced signals being deteriorated.
It is presupposed that the cause of deterioration of reproduced signals is the moisture in the air that reaches either or both of the recording and reflective films and generates a chemical reaction to the films or between the films and neighboring films. Deterioration of the recording and/or reflective films gives adverse effects to data recording to the recording film.
One solution to solve the problem caused by the moisture in the air that penetrates into an optical storage medium is disclosed, for example, in Japanese Un-examined Patent Publication No. 2006-294169 (referred to as document 1, hereinafter). The solution is to provide an intermediate layer for adjusting moisture permeability between a recording film made of an organic material and a cover layer. The intermediate layer contains an oxide or a nitride including at least one of the three elements: Nb (niobium), Al (aluminum) and Si (silicon), or a sulfide including Zn (zinc).
The experiments done by the inventors of the present invention for the evaluation of the long-term reliability to sample optical storage media having an inorganic recording film with an intermediate layer made of Nb2O5 showed a higher average SER after the sample media were left in a hot and humid environment.
Evaluated in the document 1 are jitters in reproduction before and after the optical storage media were left in a hot and humid environment. Suppression of jitters only is not enough to meet the demand for longer-term reliability for optical storage media, particularly, those having a recording film made of an inorganic material.